South Pole Telescope makes new maps of microwave sky which lead to amazing discovery

An international team of astronomers has found an extraordinary galaxy cluster, one of the largest objects in the Universe, breaking several important cosmic records. Observations of this cluster, taken with NASA's Chandra X-ray Observatory, the US National Science Foundation’s South Pole Telescope, and eight other world-class observatories, may force astronomers to rethink how these colossal structures, and the galaxies that inhabit them, evolve.

The team, including researchers from McGill University, report in the journal Nature that the Phoenix Cluster, named for both its location in the constellation of the Phoenix and because of its remarkable properties, is located about 5.7 billion light years from Earth. The object is the most powerful producer of X-rays of any known cluster, and among the most massive. The data also suggests that the rate of hot gas cooling in the cluster’s central regions is the largest ever observed.

"The mythology of the Phoenix - a bird rising from the dead - is a great way to describe this revived object," explains Michael McDonald, a Hubble Fellow at the Massachusetts Institute of Technology and lead author of the paper. "While galaxies at the center of most clusters may have been dormant for billions of years, the central galaxy in this cluster seems to have come back to life with a new burst of star formation."

Like other galaxy clusters, Phoenix contains a vast reservoir of hot gas - containing more normal matter than all of the galaxies in the cluster combined - that can only be detected with X-ray and microwave telescopes like Chandra and the South Pole Telescope. The prevailing wisdom had once been that this hot gas should cool over time and sink to the galaxy at the center of the cluster, forming huge numbers of stars. However, most galaxy clusters have formed very few stars over the last few billion years.  Astronomers think that the supermassive black hole in the central galaxy of clusters pumps energy into the system, preventing cooling of gas and stifling star formation.

Because of their tremendous size, galaxy clusters are crucial objects for studying cosmology and galaxy evolution. Remarkably, the Phoenix Cluster and its central galaxy and supermassive black hole are already among the most massive known objects of their type. Finding one with such extreme properties like the Phoenix Cluster is important for our understanding of how these objects and their constituent galaxies evolve.

“We had to cast a wide net to find such a unique cluster,” said co-author McGill Professor Gil Holder, Department of Physics. “The Phoenix was found amid hundreds of new galaxy clusters discovered using the South Pole Telescope as part of a project aimed at studying dark energy, the material that is driving the accelerating expansion of the universe”. Key team members including Keith Vanderlinde, a McGill postdoctoral fellow, and graduate students Tijmen de Haan and Jon Dudley played important roles in finding the Phoenix Cluster and estimating its properties. It was discovered in a new low-noise high-resolution map of 1/16 of the microwave sky using advanced statistical tools and techniques that were developed at McGill.

The frenetic pace of star birth and cooling of gas in Phoenix are causing both the galaxy and the black hole to add mass very quickly - an important phase that the researchers predict will be relatively short-lived. "The galaxy and its black hole are undergoing unsustainable growth," said co-author Bradford Benson, of the University of Chicago.  "This growth spurt can't last longer than about a hundred million years, otherwise the galaxy and black hole would become much bigger than their counterparts in the nearby Universe."

"This spectacular star burst is a very significant discovery because it suggests we have to rethink how the massive galaxies in the centers of clusters grow," said Martin Rees of Cambridge University, who was not involved with the study.  "The cooling of hot gas might be a much more important source of stars than previously thought."

The Phoenix Cluster was originally detected by the South Pole Telescope, and later was observed in optical light by the Gemini Observatory in Chile as well as the Blanco 4-meter and Magellan telescopes, also in Chile.  The hot gas and its rate of cooling were estimated from Chandra data. To measure the star formation rate in the Phoenix Cluster, several space-based telescopes were used including NASA's WISE and GALEX, and ESA's Herschel.

NASA's Marshall Space Flight Center in Huntsville, Ala., manages the Chandra program for NASA's Science Mission Directorate in Washington. The Smithsonian Astrophysical Observatory controls Chandra's science and flight operations from Cambridge, Mass. The work conducted at McGill was funded by NSERC and the Canadian Institute for Advanced Research.